1. Field of the Invention
This invention relates to mechanical resonator devices having phenomena-dependent electrical stiffness.
2. Background Art
Recent advances in micromachining technology that yield high-Q micro-scale mechanical resonators may soon enable substantial size and cost reductions for the highly stable oscillators used in communication and timekeeper applications. In particular, IC-compatible surface-micromachined mechanical resonators from MF to VHF frequencies with Q's in excess of 10,000 have been demonstrated in polycrystalline silicon structure materials as described in C. T. -c. Nguyen, “Frequency-Selective MEMS for Miniaturized Low-Power Communication Devices (invited),” IEEE TRANS. MICROWAVE THEORY TECH., Vol. 47, No. 8, pp. 1486-1503, August 1999.
Prototype high-Q oscillators feature micromechanical (or “μmechanical”) resonators integrated together with sustaining electronics, all in a single chip, using a planar process that combines surface-micromachining and integrated circuits, have also been demonstrated as described in “C. T. -C. Nguyen and R. T. Howe, “An Integrated CMOS Micromechanical Resonator High-Q Oscillator,” IEEE SOLID-STATE CIRCUITS, Vol. 34, No. 4, pp. 440-445, April 1999.
Unfortunately, although the Q of the resonators in these oscillators is sufficient to garner respectable short-term stability, their thermal stability falls well short of the needed specifications, typically exhibiting frequency variations on the order of 1870 ppm over a 0° C. to 85° C. range, as shown in FIG. 1, which compares the performance of a polysilicon folded beam μmechanical resonator with that of AT-cut quartz. Although techniques exist to alleviate this thermal dependence (e.g., temperature compensation circuitry, or oven control), all of them consume significant amounts of power, and thus, reduce the battery lifetime of the portable devices.
The above-noted pending application entitled “Micromechanical Resonator Device” discloses a geometric stress-compensated device that utilized strategic geometrical design of a resonator and its support structure to introduce temperature-dependent stresses on its resonator beam that counteract temperature-induced frequency shifts caused largely by Young's modulus temperature dependence.
In the article entitled “Geometric Stress Compensation for Enhanced Thermal Stability in Micromechanical Resonators,” W. -T. Hsu et al., ULTRAS. SYMP., 1998, pp. 945-948, a geometric stress-compensation design technique is disclosed with respect to low-frequency (L F, e.g., 80 kHz) nickel folded-beam μmechanical resonators that used a geometrically-tailored stress-versus-temperature function to cancel the thermal dependence of the material Young's modulus, resulting in an overall lower frequency excursion over a given temperature range, and generating zero temperature coefficient TCfo points in the process.
Other related articles include: C. T. -C Nguyen, “Micromachining Technologies for Miniaturized Communication Devices,” PROCEEDINGS OF SPIE: MICROMACHINING AND MICROFABRICATIONS, Santa Clara, Calif., Sep. 20-22, 1998, pp. 24-38; Kun Wang et al., “VHF Free-Free Beam High-Q Micromechanical Resonators,” XP-000830790, Jan. 17, 1999, pp. 453-458; and C. T. -C Nguyen, “Frequency-Selective MEMS For Miniaturized Communication Devices,” IEEE, 1998, pp. 445-460.